A fuel cell is an electrochemical energy conversion device. It converts energy from the chemical reaction of the fuel and the oxidant into electrical energy. Fuel cells have been proposed for use in both stationary applications, such as power plants, as well as smaller, portable applications, such as electrical vehicular power plants, which would replace the internal combustion engine in automobiles. The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells, also called a stack, commonly arranged in series.
In proton exchange membrane (PEM) fuel cells, hydrogen or hydrogen containing gas is supplied as the fuel to the anode of the fuel cell and oxygen or oxygen containing gas, for example air, is supplied as the oxidant to the cathode of the fuel cell. With particular reference to FIG. 1, a mono-cell PEM fuel cell 10 is shown having a membrane electrode assembly 12 sandwiched between a pair of electrically conductive electrode plates 14. This configuration is repeated in series to form PEM fuel cell stacks, where the plurality of mono-cells is separated from each other by bipolar electrode plates. The plates 14 may be made of a composite of conductive materials and polymer binders, graphite, or corrosion resistant metals. The membrane electrode assembly (MEA) 12 and electrode plates 14 are clamped together between rigid end plates 16. The electrode plates 14 each contain a plurality of lands 18 defining a plurality of flow channels 20 that form a flow field 22 for distributing reactant gases, for example, H2 and O2, to opposing faces of the MEA 12. In the case of a multi-cell PEM fuel cell stack, a flow field is formed on either side of the bipolar plate, one for the H2 and one for the O2. Nonconductive gaskets 24 provide seals and electrical insulation between the several components of the fuel cell 10. Electronically isolated bolts, not depicted here, extend through holes located at the corners of the several components for clamping the PEM fuel cell 10 together.
With particular reference to FIGS. 2 and 3, the MEA 12 includes a membrane 26 sandwiched between an anode catalyst layer 28 and a cathode catalyst layer 30, an anode diffusion media 32a and a cathode diffusion media 32b. The anode diffusion media 32a and the cathode diffusion media 32b can be referred to generically and collectively as a gas diffusion layer (GDL), or as a gas diffusion media (GDM). As shown, H2 flow channels 20a that form the anode side H2 flow field, lie immediately adjacent the anode diffusion media 32a and are in direct mutual contact. Similarly, O2 flow channels 20b that form the cathode side O2 flow field lie immediately adjacent the cathode diffusion media 32b and are in direct mutual contact. FIG. 4 illustrates the spatial relationship between the channels and the lands and shows their distribution over the face of the electrode plates.
In operation, an H2 rich stream flows into an inlet side of the anode side flow field and concurrently, an O2 stream (or alternatively air) flows into an inlet side of the cathode side of the flow field. H2 flows along the anode side of the MEA 12 and the presence of the anode catalyst 28 causes the H2 to dissociate into hydrogen ions (H+) with each hydrogen atom contributing an electron. The electrons travel from the anode side, through the bipolar plate to the cathode of the adjacent cell, closing an electric circuit, not shown, which thereby may be used to perform work. The membrane layer 26 is a selective medium which allows positively charged protons to flow through but does not allow the negatively charged electrons to flow through. Thus, the H+ions can flow directly through the membrane to the cathode catalyst 28. At the cathode side, the protons combine with oxygen atoms and the electrons flowing through the electric circuit, forming water, H2O. These processes are typically occurring as the reactants flow through their respective flow fields. This results in a pooling of water near the outlet side of the reactant gas channels.
Usage of the GDL 32 is desirable because it allows for relatively even distribution of the reactants in the active area of an operating fuel cell. This distribution is accomplished through the diffusion of the reactants from the flow channels 20 through the GDL 32 and into contact with their respective catalysts thereby facilitating the required reactions. The GDL 32 also assures good electrical contact across the fuel cell stack.
The GDL 32 also facilitates the back diffusion of the primary product of the fuel cell reactions, namely H2O. The redistribution of H2O across the PEM fuel cell 10 is of critical importance in the performance of the cell. A more uniform hydration of the PEM fuel cell 10 allows for better contact between the reactants and the electrodes, and therefore betters performance.
The use of the GDL 32 improves the performance and stability of the PEM fuel cell 10. The GDL 32 is sufficiently permeable to reactant gases and liquid water under the lands 18 and between the flow channels 20. The electrical conductivity of the GDL 32 is sufficiently high allowing the transport of the electrons over the flow channels 20 between the lands 18 and the MEA 12.
The most commonly used diffusion media material of the GDL 32 is carbon fiber paper, such as for example made by Toray of Japan, Specracorp of Massachusetts, and SGL of Germany. Each of these products is produced through similar processes. Carbon fibers are dispersed in water, before being drawn across a uniform surface. The surface is often a wire mesh or a drumhead apparatus. The gas diffusion media material is allowed to dry and then is ready for further processing.
It is known in the art that the GDL 32 will take on a compressive set, which is a geometrical distortion of the GDL thickness under pressure over the course of the operation of a PEM fuel cell 10. In this regard, FIG. 5 shows a graph 40 of stack thickness change versus number of operation cycles for two fuel cell stacks with two different “as-received” commercial GDL materials, represented by plot 42 and plot 44. In both stacks, the stack thickness change vs. initial value increases as the number of operating cycles increases. As the number of operating cycles gets sufficiently high, the thickness change per cycle decreases, and it eventually reaches a steady state. Plot 42 is for a GDL material which is relatively soft and generally more prone to adopting a compressive set as compared to plot 44 for a GDL material which is relatively firm and less flexible. This results in the GDL material of plot 42 having larger stack thickness change than the GDL material of plot 44, and, thus, requiring more cycles of operation before it reaches a steady state, where no further thickness change occurs.
One of the consequences of the compressive set is that it may result in significant loss of compression pressure in the fuel cell stack. Loss of compression pressure will cause an increase in contact resistance and thereby degrade the performance of the fuel cells, particularly when high power output is needed.
Another consequence of the compressive set of the GDL material is an intrusion of the material into the flow channels 20. Referring to FIG. 6, the GDL 32 has an intrusion 46 into the flow channel 20 having a magnitude of GDL intrusion displacement 48. Due to the variability of the thickness and/or of the compressive mechanical properties of the GDL 32, the magnitude of GDL intrusion displacement 48 will vary from cell to cell or even within the same cell. A consequence of the intrusion 46 is that it can significantly alter the flow distribution of the reactant gases. Any significant misdirection of the reactants within the fuel cell stack will cause power instability and it will require operation under elevated stoichiometric conditions, which is disadvantageous for the fuel cell. It is contemplated that by reducing the magnitude of intrusion displacement 48, the sensitivity of reactant gases flow distribution to the variability of GDL thickness and compressive mechanical properties can be reduced.
In order to prevent fuel cell compression loss over time, three principal strategies have been developed in the prior art. A bladder type compression device has been used to maintain a constant stack compression force; however, this device is bulky and not useful for automotive applications. Recompression of the stack as part of a standard maintenance regimen can reset the condition; however, this process requires removal of fuel cell stacks from the system and does nothing to improve the non-uniform nature of the intrusions into the reaction channels. Lastly, staged compression assumes the compression load will increase in several steps with a few stack operation hours between such steps until the compression pressure reaches a nominal value; however, while further compression loss is prevented and the GDL intrusion problem is partly solved, it creates an unacceptable delay in the placing of the stacks into the system.
What remains needed in the art, therefore, is a method that would reduce non-uniformity of compressive mechanical properties between GDL sheets, lessen the intrusion of the sheets into the flow channels, and eliminate the loss of compression during the life of the stack.